I read that astonishing figure in the paper today...

I found it really depressing! As I mentioned in my "Poule au Pot" post last week, I was brought up on saving and re-using leftovers mroe often than not. And without going to extremes like keeping 1/4 of a dryish lemon in a Tupperware for months in the fridge (don't laugh, my mum does that)  we do pretty well I hope.
One dish I find brilliant to finish leftover roast beef is my mum's tarte a la viande (meat tart) . It was a regular at the dinner table on a Monday night. I haven't made it for ages, but simply because we tend to eat more casserole beef at the moment.
 All you need is:

1 shortcrust pastry
 a few slices of cold roast beef (5 or 6)
1 can of chopped tomatoes
 a large handful of parsley
1 onion
 salt and pepper
1 egg

Roll your pastry to 1 cm deep
line a tart dish with greasproof paper and place your pastry on top
Roughly cut the beef in strips
Transfer into a food processor and chop finely
Finely chopped the onion
Gently fry in a little olive oil unti soft but not coloured
Pour the meat mince into a large bowl
add the finely chopped parsley, the egg and the onion
Season with a little salt and lots of freshly crushed pepper
spread the  mixture on the pastry
Bake in a hot oven (190) for 25 minutes until set

it's very nice served warm but also brilliant cold for a pic nic (yeah, maybe not right now but the sun will come back!)

here are a few more ideas for using leftover...

Chicken- 
Waterzooi soup

Cheese-
Goat's cheese-  classic goat cheese salad
Gruyere, cheddar, comté, any hard cheese really- Croutes au fromage

Stale bread, brioche, panatonne, croissant or even pain au chocolat-
Pain perdu

Eggs-
Spinach and boiled eggs gratin
Swiss chard quiche
pain perdu again, even if you make it with fresh bread

sausage meat or an open pack of sausages
Chou farci (stuffed cabbage)
saucisson Beaujolais

bon Apetit et a bientot!

PA: For more repices with leftovers why not give a try to http://lesrecettesdelorette.blogspot.com/